Field of the Invention
The present invention relates to a magnetron with reduced dark current. More particularly, it concerns a magnetron that can suppress increases of dark current to raise the oscillation efficiency thereof, with little noise radiated out, for use in a microwave oven. By "noise" is meant microwaves outside the frequency range of 2400 to 2500 MHz, including side lobes and spurious signals.
FIG. 6 depicts a side view cross-sectioned in an axial direction of a tube illustrating an example of a prior magnetron. The example has cavity resonators formed of vanes 3 extended radially inwardly from an anode cylinder 2 made chiefly of copper. A cathode 1 is positioned at the center of the anode cylinder 2. There is formed an annular interaction space around the cathode 1. The anode cylinder 2 and the vanes 3 are integrated together by way of hobbing or are fabricated individually before being brazed together. The cathode 1 is usually made of helically coiled thoriated-tungsten wire. Both ends of the cathode 1 are connected and held with end shields 12 and 13, respectively. An antenna 5 is connected with a microwave output port 8 to withdraw the microwave energy out from one of the vanes 3. Pole pieces 4 are attached to the upper and lower ends of the anode cylinder 2, respectively, to efficiently concentrate lines of magnetic force into the interaction space formed between the tips of the vanes 3 and the cathode 1. The ends on the interaction space side of the peripheral portions of the end shields 12 and 13 are displaced along the tube axis from the axial ends of the vanes 3 near the cathode 1 toward the outside of the interaction space.
An axial static magnetic field is formed in the interaction space defined by annular permanent magnets 6 which are short in axial length and which are provided at the top and the bottom of the anode cylinder 2, respectively, as a source of a magnetomotive force, a yoke 7, which enclose the tube and is in contact with the outsides of the permanent magnets 6 to form an external magnetic circuit, and the pole pieces 4, each of which has a peripheral portion sandwiched between the adjacent end of the anode cylinder 2 and the respective adjacent permanent magnet 6 and has an inner portion extending close to the end of the interaction space. A vacuum envelope, including the anode cylinder 2, is grounded for safety. The cathode 1 has a high negative d-c potential applied thereto. Electrons are attracted from the cathode 1 toward tips of the vanes 3 which are at ground potential, and at the same time are acted upon by the axial static magnetic field in the interaction space and have a force exerted thereon in a direction perpendicular to the direction of the magnetic field and the motion thereof, and some electrons are turned back toward the cathode 1 after whirling near the tips of the vanes 3 circumferentially about the cathode.
This causes a high-density area and a low-density area of electrons to be formed in the interaction space. An electron cloud of the high density whirls at a high speed in the interaction space to excite microwave oscillations in a group of cavity resonators formed by the anode cylinder 2 and the adjacent vanes. Of the electric oscillations in the group of cavity resonators, the so-called .pi. mode of oscillation having a reversed phase between adjacent cavities is strongest and stablest. To add further stability of the oscillation in the .pi. mode, an inner strap ring 10 and an outer strap ring 11 for connecting alternate ones of the vanes 3 are contained in grooves at the axial ends of the vanes for the purpose of tying together points at the same potential (same phase). The microwave energy is withdrawn through the microwave output port 8 by the antenna 5 mounted at the end of one of the vanes into a microwave oven for heating food, for example. Note that the cathode 1 is supported by cathode stem 9 through heater-current-feeding wires.
Such magnetrons are mostly used for microwave ovens at present. Beside characteristic performance, an important factor of home appliances is low price. For this purpose, magnetrons for use in microwave ovens have been designed to lower their cost in a variety of ways. The prior magnetron described above has strap rings at the ends of the vanes on both the microwave output port 8 side and the cathode stem side. If the strap rings can be reduced to only one at one of the ends of the vanes, the number of manufacturing steps and parts cost can be decreased.
However, if a strap ring is provided only at one of the axial ends of the vanes and the other portions are left as they were, the resonance frequency of the cavity is made too high. This is caused by the fact that the electric capacitance between the inner and outer strap rings and the capacitance between a vane connected with one of the two strap rings and the other of the two strap rings are almost halved. A method of suppressing the increase of the resonance frequency is proposed in the Japanese Patent Laid-Open No. 4-223026. To electrically connect alternate ones of the vanes, the inner and outer strap rings are fitted in the grooves only at the ends of the vanes on the microwave output port 8 side at an equal interval from the tube axis, and also, each vane has an extended portion symmetrically projected toward the tips of the adjacent vanes and having surfaces in parallel with corresponding surfaces of an extended portion of the adjacent vanes, thereby compensating for reduction of the electric capacitance. Another method of suppressing the increase of the resonance frequency is to decrease the axial width of each vane so as to increase the inductance of the vane. If the two above-described methods are used together, the end of each vane on the cathode stem side has a level difference provided at a boundary between the extended portion and a portion adjoined with it to narrow the axial width of the portion of the vane other than the extended portion, thereby increasing the inductance of the portion of the vane other than the extended portion. Such a combined method can increase the inductance and the electric capacitance of the cavity resonator. Even when the strap rings are provided only at one of the axial ends of the vanes, therefore, the resonance frequency can be made the same as in the prior art. FIG. 7 (a) depicts a plan view illustrating the anode cylinder of the magnetron of the above-mentioned technique, and FIG. 7 (b) depicts a cross-sectional view illustrating the same anode cylinder. In FIGS. 7(a) and 7(b), reference numeral 31 denotes the extended portion of the vane 3, while reference numeral 3a is the groove for fitting in the strap ring, and reference numeral 3b is the level difference at the boundary between the extended portion and the portion adjoined with it. The vanes 3 and the anode cylinder 2 described above are integrally fabricated by way of hobbing. A groove (not marked) for mounting an end of the antenna has to be actually made only on a single vane. But, the grooves are formed in all the vanes as they are made at one time by a lathe.
Integral forming of the above-described anode cylinder of the magnetron by way of hobbing makes the shape of the hob somewhat complicated. But, as an amount of the material to be extruded at the center of the material blank is reduced, resistance to the hob is reduced, thereby making longer the service life of the hob.